Greengrass:
First rule of Service life - always have a plan B. Your son is best off doing the degree he wants to do, not the degree he thinks will please OASC. The reality is that his chances of making RAF aircrew were slim and are now slimmer, so he needs a degree that will be useful to him later in life, or elsewhere in the RAF (or the RN/Army).
As it happens there is no "perfect" aircrew degree subject - they need to be numerate, but don't need a Maths degree, they need to be able to write well enough, but don't need a degree in English. They need leadership skills, but a management degree won't help. A foreign language is no bad thing - see if you can work out who we will be fighting in ten to twenty years time, perhaps.
I knew a capable Dark Blue pilot with a degree in theology, which always seemed a good choice - even if he didn't believe in God, he might have understood his mysterious ways